1. Field of the Invention
The present invention relates to an improved screed for use on street paving machines. More particularly, the present invention relates to a screed that has a longterm service life, that retains heat for an extended period of time, and that is readily affixable to standard paving equipment. The present invention is an improved screed formed of a chromium-carbide composition alloy for extremely high wear-resistance and low thermal conductivity, with a low-carbon steel cladding to provide reliable screed-to-paver attachment.
2. Description of the Prior Art
In the road construction industry, the paving machine is the most commonly used piece of equipment for placing paving material onto a graded underlying surface. The paving machine comprises, among many other components, a screed. The screed provides an initial mechanism for leveling hot, relatively soft material, such as asphalt, deposited on the underlying surface by the paving machine. The screed has an essentially smooth bottom surface contacting the paving material and is affixed to structural members of the paving machine by its top surface. In operation, the paving material is first deposited by the machine as the machine travels along a predetermined path. The screed is affixed to the paving machine at a point behind the opening through which the paving material is deposited, and is drawn over the deposited material to create a relatively smooth, level surface before the hot material cools and hardens. It is necessary to keep the screed temperature at or near the temperature of the hot paving material in order to prevent the material from sticking to the screed as the screed passes over and contacts it.
Screeds are as well known in this field as the paving machines themselves. Modifications in their design have primarily been related to dimensional restrictions. In particular, the patents issued to Birtchet (U.S. Pat. No. 3,673,930) and Davin (U.S. Pat. No. 3,702,578) disclose means for adding extensions in order to increase the operational width of the paving machine. Davin specifically provides a means for extending the screed length in a short period of time. Such a goal is of considerable importance in this labor-intensive industry for it is well known that time spent waiting for equipment to be modified, or for equipment to reach its operating parameters, is lost time.
One other improved screed, related to the present invention, is disclosed in the patent issued to Lutz (U.S. Pat. No. 4,865,487). Lutz describes the need to provide a paving screed that has a high-wear-resistant bottom plate. This is achieved by fabricating the bottom plate of steel with a hardness in the Brinell range of about 450-500. Most conventional paving machines utilize a mild steel screed, Brinell hardness in the range 150-200, that must be replaced in a relatively short period of time, due to the abrasiveness of the paving material and the underlying surface. On the other hand, the hard steel used by Lutz is believed to extend the service life of the screed considerably. Furthermore, Lutz provides curved leading and trailing edges, formed of low-carbon steel that is welded to the flat hard steel screed, that make the screed reversible for even greater service life.
While the screed described by Lutz is theoretically better than the mild steel screeds used previously in that it provides better wear resistance, there are several deficiencies that make it unsuitable for actual use. Of notable concern is that the entire screed--except for edge ad-ons--is fabricated of the hard steel. Threaded bolts, used to attach the screed to a mounting assembly, must be affixed to the top surface of the screed--that is, the surface nearest the mounting assembly and farthest from the deposited paving material. Although the bolts are typically made of mild steel, Lutz notes that stainless steel bolts may also be used. The most effective and reliable means for attaching the bolts is by welding them to the top screed surface. However, welding a dissimilar metal to the hard steel in a localized area is extremely difficult and often results in severe degradation of the hard steel in that localized area. This degradation is most readily observed when the bolts are torqued to secure the screed to the mounting assembly; under the required torque loading the bolts often break away from the hard steel surface. Similar catastrophic failure may result at the interface of the welded leading and trailing edges of the screed described.
Another problem related to the use of hard steel for screeds is the thermal conductivity of steel. As previously indicated, it is necessary to keep the screed temperature at or near the paving material temperature in order to facilitate smooth leveling of the material. This is done by heating the screed using conventional heat transfer means. At the start-up, both the mass of paving material within the paving machine and the screed are heated to operating temperature. This operating temperature varies from one region of the country to another and is a function of the paving material and ambient conditions. Whenever the paving operation is stopped for an extended period of time as, for example, at lunchtime, the machine is shut down. The paving material, with its low thermal conductivity and considerable mass will cool only slightly. However, the steel screed cools much more rapidly. As a result, at the end of a break there is additional down-time caused by the need to bring the screed back up to operating temperature.
Still another problem associated with a hard steel screed is the fact that this material, while an improvement over the mild steel of prior screeds, is still subject to considerable wear. The severity of the environment within which the screed operates and the need to minimize down-time in replacing or reversing screeds requires the use of a material that will withstand particularly difficult abrasion. Therefore, what is needed is an improved screed that is formed of a material that has extremely high wear resistance. What is also needed is an improved screed that is readily adaptable to conventional paver mounting assemblies without catastrophic failure of mounting components. Further, what is needed is an improved screed with relatively low thermal conductivity in order to minimize start-up time after long stoppages.